1. Field of the Invention
The present invention relates to a method of manufacturing a rotating device and a rotating device, and more specifically, a technology that improves the level of cleanness of a component part of the rotating device, thereby improving the product quality.
2. Description of the Related Art
Recently, the rotational accuracy of rotating devices like HDDs has been remarkably improved by causing those devices to have a fluid dynamic bearing unit. Together with such an improvement, rotating devices like HDDs are required to have a higher density and a more increased capacity. For example, an HDD that magnetically records data rotates a recording disk formed with recording tracks at a high speed, and executes a read-write operation of data while allowing a magnetic head to trace the recording tracks with a slight flying height therebetween. In order to cause such an HDD to have a high density and an increased capacity, it is necessary to make the width of recording track narrowed. Together with narrowing of the track width, it becomes necessary to make a space between the magnetic head and the recording disk further narrowed. There is a requisition to cause the flying height between the magnetic head and the recording disk to be extremely narrow which is equal to or smaller than, for example, 10 nm in consideration of the read-write reliability of data.
In order to accomplish the high density, magneto-resistance effect devices (hereinafter, referred to as MR devices) are frequently used for the magnetic head. Conversely, the use of the MR devices in an extremely narrow flying height brings about a serious problem which is the occurrence of thermal asperity failure (hereinafter, referred to as a TA failure) and a head crash failure. Momentary heat is generated in the MR device by kinetic energy originating from minute foreign substances on the disk surface contacting the MR device while the magnetic head is flying and tracing the recording tracks. The TA failure causes the resistance of the MR device to momentarily fluctuate and be superimposed on a reproduction signal as noises when the MR device is momentarily heated and cooled, and thus reducing the reading accuracy of the reproduction signal. The inventors of the present invention keenly studied and learned that TA failure is caused by foreign substances (hereinafter, referred to as particles) originally adhering to a rotating device and having a size of 0.1 μm to several μm or so which stick to the recording disk surface due to vibration, air flow, and the like.
Meanwhile, the rotating device includes an assembly that has a base member, a bearing unit on the base member and including a sleeve and a shaft rotatable relative to the sleeve, and a rotating body that is rotatably supported on the base member by the bearing unit. A recording disk is mounted on a hub member that is a part of the rotating body, and the rotating device is thus manufactured which includes a magnetic head, a drive device thereof, a control circuit and other necessary components.
In the case of rotating devices, an example cause of producing particles is that the rotating device is assembled with cutting and a cutting lubricant produced when the base member is machined being adhering to the base member as it is.
The base member includes a bearing hole where the bearing unit is attached, a tap hole for attaching a cover to the outer wall of the base member by means of a screw, and another tap hole for fastening the rotating device to another device by means of a screw, and the like. Such a base member is cleaned after the bearing hole and the tap holes are formed, and then the bearing unit and the rotating body are assembled therewith. Conventionally, a manufacturing method of such a rotating device is executed through a so-called batch scheme that independently executes a process of forming the bearing hole in the base member, a process of forming the tap holes in the base member, a process of cleaning the base member and a process of assembling the bearing unit and the rotating body with the base member. It is typical that intermediate products are stored before and after each process according to the batch-scheme manufacturing method. When the intermediate products are stored, the intermediate products may be scratched or particles may stick thereto during storing. Moreover, when a cutting lubricant is used in the process of forming the bearing hole and the tap holes, such a lubricant may be dried and solidified while the intermediate products are stored. Once the cutting lubricant becomes solidified, elimination thereof is not easy and the lubricant remains on the base member without such a lubricant being eliminated sufficiently, and the remaining lubricant may become particles.
Conventionally, the base member is assembled together through a cleaning process of eliminating particles like dust with the aid of, for example, ultrasound in a cleaning tank filled with a predetermined cleaning liquid. Regarding the cleaning process, a so-called batch cleaning is popular in which base members in a unit of several tens are put in a cleaning basket and soaked in the cleaning liquid. Such a cleaning process is often applied when small components are mass-produced. According to the batch cleaning, the base members are dried after cleaning together with the cleaning basket, and are stored in a storage like a warehouse. Next, in the assembling process, the stored base members are taken out and supplied to an assembling line, and thus a rotating device is assembled (see, for example, JP H07-124529 A).
According to the above-explained manufacturing method of executing in a batch scheme the process of forming the bearing hole in the base member, the process of forming tap holes in the base member, and the process of cleaning the base members, in general, the level of cleanness as a whole by the cleaning process is low. For example, the level of cleanness of a rotating device is evaluated based on the number of particles having a size of equal to or larger than 0.5 μm and present per 1 cm2 (hereinafter, referred to as an LPC). The LPC is obtained through the following procedures. That is, a test object is soaked in a tank filled with, for example, 2000 cc of pure water, is irradiated with ultrasound of 98 w at 68 kHz for 120 seconds, and the number of particles present in the pure water is counted using a liquid particle counter like CLS-700 or LS200 made by Particle Measuring Systems, Inc., U.S.A.
Moreover, according to the conventional manufacturing method of executing in a batch scheme the process of forming the bearing hole in the base member, the process of forming the tap holes in the base member, and the process of cleaning the base members, when it is attempted to obtain a desired cleanness level, cleaning takes a long time and the working efficiency is poor. Moreover, after cleaning, particles floating in air may stick to the base member which is being stored in a warehouse. When many particles remain on the base member in this fashion, if the flying height of the magnetic head is reduced, the occurrence rate of TA failure becomes high, which disturbs accomplishment of the higher density of the rotating device and the increased capacity thereof. Hence, a process of wiping out particles using a solvent like hexane may be possibly added before and after assembling. However, it brings about reduction of the manufacturing efficiency and in many cases, elimination of the particles through the wiping process is insufficient.
The present invention has been made in order to overcome the problem explained above, and it is an object of the present invention to provide a technique of improving the cleanness level of each component of a rotating device and of maintaining the probability of occurrence of a TA failure at low even if the flying height of a magnetic head is reduced.